German Patent Publication No. (DE-OS) 2,623,385 disclose and apparatus for performing the method. The known method requires for its performance relatively complicated drawing mandrels which are interconnected with one another, for example, by plug-in connections or similarly operating connections. The structure required for the interconnection of the mandrels is not only complicated, it is also expensive.
Another disadvantage of conventionally interconnected drawing mandrels is seen in the fact that a set of such interconnected mandrels remains inside the pipe in the nozzle area until a further indentation is made. This fact requires that the spacing between individual mandrel sections and thus the size, especially the length of the intermediate connecting components, is sufficiently large in order to transport the next smaller mandrel section through the drawing station sufficiently far in the forward direction so that between two neighboring mandrel sections of different diameters a dent can be made at the drawing plate or nozzle. As a result, the space available for the denting tool is very limited. Another disadvantage of this arrangement is the fact that due to this limited space available for the denting tool, a substantial length of pipe will be drawn in which merely the diameter has been reduced by the drawing plate or nozzle but in which the wall thickness of the pipe has not been reduced. This is so because the denting can be made only when the mandrel section with the next smaller diameter has assumed the position in the zone of reduced wall thickness as produced by the preceding larger diameter drawing nozzle in the drawing plate.